For sake of discussion I reread your letters from Marx and Ambassador Adams.
I noticed first that Marx's emphasis on "the fanatic partisanship of the upper classes..." sounded like it was written by our own DiogenesLamp -- how can that be, a Lost Causer and Karl Marx seeing the world through the same class-warfare eyeglasses?
I also noticed that Adams' response was total diplomatic boilerplate -- in effect: "the United States seeks peace and good relations with all."
So I've seen nothing there to suggest that Lincoln & Marx were "friends", any more than, oh, say, Trump and David Duke today are "friends".
Obviously they weren't and anyone who poses such a proposition is a low-grade moron.